1. Field of the Invention
The present invention relates to a speaker unit for a musical instrument including an electrostatic speaker and configured to generate musical sounds based on a performance operation in a musical instrument main body. The present invention also relates to an electronic keyboard musical instrument and an electronic musical instrument including the speaker unit.
2. Discussion of Related Art
There has been conventionally known a speaker called an electrostatic speaker or a planar speaker as disclosed in the following Patent Literature 1. The electrostatic speaker is constituted by two electrodes which are opposed to each other with a spacing interposed therebetween and a sheet-like oscillating member having electric conductivity and interposed between the two electrodes. When a voltage applied to the electrodes is changed in a state in which a suitable bias voltage is applied to the oscillating member, an electrostatic force that acts on the oscillating member is changed, whereby the oscillating member is displaced. By changing the application voltage in accordance with acoustic signals to be inputted, the oscillating member is repeatedly displaced, namely, the oscillating member oscillates, so that reproduced waves in accordance with the acoustic signals are generated from the oscillating member.
Since the electrostatic speaker is inferior in acoustic characteristics in the bass range, the electrostatic speaker is not employed positively as a main speaker in electronic musical instruments. The electrostatic speaker, however, has characteristics that a cone speaker does not have.
For instance, in the electrostatic speaker, the larger the area of a planar plate of the electrostatic speaker, the stronger the sound directivity in a direction perpendicular to the planar plate. Consequently, the electrostatic speaker offers a property ensuring that sounds reach far. Further, in the electrostatic speaker, sounds can be emitted from both surfaces thereof. In particular, a thin and flexible electrostatic speaker is small in size and light in weight, and has flexibility that enables the electrostatic speaker to be curved or twisted, ensuring a freedom in shape.    Patent Literature 1: JP-A-2010-68053    Patent Literature 2: JP-A-2003-263165    Patent Literature 3: JP-A-2003-173185
In the meantime, with regard to the cone speaker, there has been known an electronic musical instrument in which an orientation of the cone speaker is variable relative to an instrument main body (Patent Literatures 2 and 3).